Cultivating sage for a long time (4)
I believe the answer is:
ages
'long time' is the definition.
(I've seen this before)
'cultivating sage' is the wordplay.
'cultivating' is an anagram indicator.
'sage' with letters rearranged gives 'AGES'.
'for a' is the link.
(Other definitions for ages that I've seen before include "Indefinitely long time" , "Historic periods or prolonged time" , "Aeons" , "Long periods" , "Epochs" .)